This invention concerns a pulse modulation method for pulse modulating binary codes, and in particular, a pulse modulation method used in an infrared communication device such as an IR module or remote control transceiver. Pulse-position modulation (PPM) is commonly used in remote control devices that control audiovisual equipment by infrared rays. PPM uses constant-amplitude, constant width pulses whose timings relative to a clock are proportional to sampled values of the modulating signal. In a digital signal, such as a binary code, direct conversion is used instead of sampling.
Referring to FIGS. 7(a)-7(c) and FIG. 8, an ID code and a data code, each consisting of an 8-bit word, are shown. The ID code, which is determined by the attributes of a controlled device (not shown), prevents misoperation with other devices. Devices meant to interoperate share identical ID codes. The data code is an encoding of the control data that controls the controlled device. In these codes, a pulse stop interval Tr, i.e., the interval between pulses, is determined by the value of each bit and is PPM-modulated into a signal such as that shown in FIG. 7(b).
Referring also to FIG. 8, when each bit is "0", a pulse of pulse width t is generated while pulse stop interval Tr is set to an interval that is the same as pulse width t. When the bit is "1", a pulse of pulse width t is generated while pulse stop interval Tr is set to an interval of 2t.
A header signal is added before this modulated signal. The header signal consists of a pulse of pulse width t and a 3t-long pulse stop interval Tr. An end signal, added after the modulated signal, consists of a pulse of pulse width t and a 5t-long pulse stop interval Tr, producing the PPM-modulated signal shown in FIG. 7(b). The header signal alerts the receiving circuitry to the beginning of the transmission and facilitates demodulating the modulated signal sent subsequently. The end signal signals the termination of the transmission of the modulated signal to the receiving circuitry. A transmission frame interval Tf is the length of time required to transmit the header signal followed by the ID code, the data code, and the end signal.
In order to prevent mixing of signals with other devices and misoperation, this PPM-modulated signal is secondarily modulated by a carrier modulation wave of about 38 kHz as shown in FIG. 7(c) for sending via infrared to the controlled device.
Referring to FIGS. 9(a)-9(c), when bits are pulse-modulated for transmission at high speed, a modified frequency modulation (MFM) method is adopted. In this MFM modulation method, a pulse "D" is generated in the center of the bit cell when the bit is "1". When the next bit is "0", a clock pulse "C" is generated at the beginning of the second and subsequent bit cells, after which the signal is inverted in the position of the pulses "D" and clock pulses "C", producing the MFM-modulated signal shown in FIG. 9(c).
Referring to FIG. 9(d), a pseudo-MFM modulation method was developed by the applicant in which the pseudo-MFM modulation signal is created based on the MFM modulation signal generating a pulse waveform at the leading edge and trailing edge of the pulse. However, this method has the drawback that a single bit cannot be demodulated by itself, but must be demodulated by comparing it with the immediately preceding bit. As in the MFM modulation method, the pseudo-MFM method requires a register on the receiver side and complicates the demodulation algorithm.
Referring to FIG. 10, a block diagram depicts a conventional transmitter 100 for sending control codes to a receiving device 200 via an infrared signal 120. A control code, called from a code register 102 by means of a keyboard 101a, a switch array 101b and a key input 101, is sent along with an ID code (not shown) to a pulse modulation circuit 104 via an output control circuit 103. In pulse modulation circuit 104, the control code/ID code is either PPM-modulated or MFM-modulated as described above. The modulated control code/ID code is secondarily modulated by the carrier frequency for transmission. An LED 105 is driven and controlled by this secondarily modulated signal to produce infrared signal 120.
An infrared PIN photodiode 106 in receiving device 200 senses infrared signal 120, performs a photoelectric conversion, and creates the received signal. The received signal is amplified by an amplifier 107, goes through a limiter level shift circuit 108, a peak detection circuit 109, and an output waveform shaping circuit 110, creating a received signal approximating FIG. 7(b) or FIG. 9(c). The bits from this received signal are demodulated by a decoding circuit 111 and sent to a high-order processing device 112 for controlling the controlled device.
Although the PPM modulation method makes demodulation easy, slow transmission speed is a problem since each bit necessarily contains pulse stop interval Tr. Transmission frame interval Tf becomes exceedingly long, thus increasing the required transmission time.
With the MFM modulation method, the unit (frame) transmission interval Tf is relatively short while the transmission speed is high. However, a single bit cannot be demodulated by itself and must be demodulated by comparing it to the immediately preceding bit. A receiving side code register is required and the demodulation algorithm is complicated, thereby requiring an expensive processor, whether software or hardware.
Referring to FIGS. 11(a) to 11(c), a rising time difference T1 and a falling time difference T2 are different in the infrared transmission pulse waveform. A pulse width PW' of the output signal whose waveform is shaped via a comparator is different from a pulse width PW of the original signal. Therefore, in the MFM modulation method, in which modulation is based on pulse width PW as well as on pulse stop interval Tr, demodulation errors are more likely to occur than with the PPM method.
Another problem with MFM is that it is subject to the effect of noise along the transmission route. As the infrared communication distance becomes greater, the transmission pulse waveform deteriorates, the pulse width PW changes similarly, and demodulation errors become more likely.
In addition, since on the transmitter side the transmission is done with a pulse train of a relatively long pulse width PW, a battery (not shown) easily runs down from driving LED 105, making the MFM method unsuitable for a portable device such as a remote control transmitter.